But California — a state that has for 50 years been a leader in environmental advocacy — is about to step into the breach. In a show of defiance, Gov. Jerry Brown, a Democrat, and legislative leaders said they would work directly with other nations and states to defend and strengthen what were already far and away the most aggressive policies to fight climate change in the nation. That includes a legislatively mandated target of reducing carbon emissions in California to 40 percent below 1990 levels by 2030.

“California can make a significant contribution to advancing the cause of dealing with climate change, irrespective of what goes on in Washington,” Brown said. “I wouldn’t underestimate California’s resolve if everything moves in this extreme climate denial direction. Yes, we will take action.”

The environmental effort poses risks. Trump and Republicans have the power to undercut California’s climate policies. “They could basically stop enforcement of the Clean Air Act and CO2 emissions,” said Hal Harvey, president of Energy Innovation, a policy research group in San Francisco.

And some business leaders warned that California’s embrace of environmental regulations could put it at a disadvantage, all the more so as conservatives elsewhere move to roll back environmental regulations.

Still, Democrats relish the prospect of challenging Trump on climate change. And California has the weight to get into the ring: It is one of the 10 largest economies in the world.

“This is not something that’s going to be fueled by dislike of Donald Trump,” said Adrienne Alvord, the western states director for the Union of Concerned Scientists. “This will be fueled by people liking these policies and wanting to see them continue. Our leadership and the people of California support the science.”

Alvord said that the fossil-fuel industry may feel emboldened to take on some of the state’s energy and climate initiatives.

“But they would be fighting a very uphill battle,” she said. “Politically, it’s going to be very difficult to really slow this train down.”